Every once in a while, the Trek franchise will stop for a moment and indulge its inner fan for a Very Special Episode that reflects upon its mythos. The most obvious way of doing this is to bring aboard TOS characters — something it did in the pilot episode (McCoy), season five's "Unification" (Spock), and would later do in Generations (Kirk) and even Star Trek 2009 (Spock again, in the rebooted alternate timeline). There seems to be an aura of legitimacy that bringing TOS forward into the future seems to bestow upon the latter-day mythos. It's a uniquely fascinating construction: Some of the actors were still playing the parts off and on even when this aired in 1992, but the time frame between TOS and TNG made the fictional TOS characters a piece of decades-old history.
In "Relics," we get Scotty, who is found having been suspended in a jerry-rigged transporter beam on a downed vessel for the past 75 years. In that time he hasn't aged a day. His ship crashed on the surface of a mythical Dyson Sphere — a massive sphere constructed by an ancient society around a star. The Dyson Sphere has a diameter equivalent to Earth's orbit around the sun. Whoa.
"Relics" is a good title, because it not only describes the mysterious Dyson Sphere, but gets to the heart of Scotty's dilemma, where he wakes up to find that he's become mostly irrelevant. Technology has moved on, most everyone who knew him is dead, and when he tries to help out in engineering, he becomes such a nuisance that Geordi finally snaps and tells him that he's in the way. (Picard, ever the wise, recognizes Scotty's plight, and appeals to Geordi to take one for the team and find a way to help a former Starfleet officer be useful again.)
Yes, the episode lays on the nostalgia fairly thick. Sometimes it is too broadly played, with a few too many examples of Scotty saying he was doing this, that, and the other while your great-grandfather was still in diapers. (One of the problems with James Doohan's take on Scotty in general is that the affects of the character are at times so overpowering that he edges close to cartoonishness.) But the scene that really works in "Relics" is when Scotty recreates the original Enterprise bridge on the holodeck. It's a terrific homage, made all the better because it's grounded in believably nostalgic dialog between Scotty and Picard about old glories that cannot be recaptured. (Scotty's drink with Data also has an elliptical quality; Data's line, "It is green," borrows from a TOS drinking scene Scotty appeared in decades earlier.)
If there's a problem with "Relics" — and I must regretfully admit that there is — it's that, apart from the holodeck sequence, the storytelling can be too earnest for its own good, and without the benefit of digging as deep as it perhaps might have. There's also an abundance of forgettable technobabble — probably a symptom of this particular period in TNG's run while also being a symptom of putting two chief engineers in the same room for long stretches of time. And I felt the awesomeness of the Dyson Sphere, a fascinating sci-fi concept, was never adequately realized. (The visual effects of the time reveal their limitations, and the sphere ultimately becomes a means to an end to supply a routine jeopardy plot.)
Still, I want to be clear that this is a pleasant and even admirable example of the Very Special Episode. It understands its characters, gives them a personal story worth telling, and uses them to drive the plot. If it doesn't achieve greatness in the process, well, so what?